This invention relates to an inductive stabilization ballast for a discharge lamp and more particularly to an improved ballast circuit including a protective device for preventing damage to the discharge lamp or to the circuit components in the event that the lamp refuses to ignite.
A known inductive stabilization ballast comprises a first pair of connecting elements between which the inductive portion of the ballast is connected, the ballast being provided with at least a second pair of connecting elements and with a resistor having a positive temperature coefficient. The discharge lamp is connected in series with the stabilization ballast. An inductive stabilization ballast of the type mentioned is disclosed in U.S. Pat. No. 3,631,322. In that known ballast the resistor having a positive temperature coefficient (PTC resistor) is disposed in proper thermal contact with the inductive portion of the ballast. In addition, the PTC resistor is connected in series with the inductive portion of the ballast. There are certain disadvantages to this type of arrangement, as will be further explained hereinafter.
If the ballast is to be used in combination with a gas and/or vapor discharge lamp which is provided with pre-heatable electrodes, two current states of the ballast can be distinguished. Namely, a first current state during the starting procedure of the lamp wherein the ballast carries the preheat current for the lamp electrodes. A second current state occurs during the operating condition of the lamp in which the lamp current flows through the ballast. In the case of a normal lamp, the duration of the first state is generally considerably shorter than that of the second current state.
As the intensity of the preheating current usually exceeds that of the current in the operating condition of the lamp, the heat generation in the ballast is greatest per unit of time in the first current state. A dangerous condition therefore occurs in a ballast if, in the absense of a protective device, the lamp does not ignite sufficiently rapidly. On the other hand, if a protective device is included in the circuit, it should be energized relatively slowly so that the electrodes of a normal lamp can be preheated to a sufficient degree without the protective device being actuated.
One of the drawbacks of the known ballast is that the PTC resistor also carries current in the operating condition of the lamp, which results in power losses. The reason for this is that the PTC resistor and the inductive portion of the ballast are connected in a series arrangement, as mentioned above. Another drawback of the known ballast is that for a relatively high ambient temperature, for example in the case of a closed luminaire, the PTC resistor might prevent continuous operation of the discharge lamp.
It is an object of the invention to provide an inductive stabilization ballast of the type mentioned in the preamble wherein the PTC resistor can be heated to its high ohmic condition only during the starting procedure of the discharge lamp.